1. Field of the Invention
The present invention relates to a forging die apparatus for processing a forging material arranged in a cavity formed by a die member so that the forging material is subjected to forging in accordance with a pressurizing action of a punch.
2. Description of the Related Art
A die apparatus has been hitherto known, in which a forging material is inserted into a cavity formed by an upper die and a lower die which are joined to one another, and a pressurizing force is applied to the forging material by the aid of a punch so that the forging material is forged to have a predetermined shape.
A cold forging die suggested by the present applicant is shown in FIG. 39 (see Japanese Laid-Open Patent Publication No. 7-178493).
The cold forging die 1 comprises a lower die 3 and an upper die 4 which are joined to one another by the aid of a clamping means 2 and which are installed on a die holder 5. A cavity 7, which is charged with a forging material 6, is formed by the lower die 3 and the upper die 4. A punch 8 is provided to pressurize the forging material 6 charged in the cavity 7.
A general forging die apparatus including the cold forging die 1 is usually designed, as shown in FIG. 40, such that a predetermined clearance A (about 0.1 mm) is provided between the punch 8 for applying the pressurizing force to the forging material 6 and a hole 9 of the upper die 4 into which the punch 8 is inserted. In other words, the hole 9 of the upper die 4, into which the punch 8 is inserted, has a diameter which is formed to be slightly larger than a diameter of the punch 8 at its portion which is inserted into the hole 9 of the upper die 4.
In this case, if the clearance A is small, the following inconvenience arises. That is, the punch 8 generates heat as the number of shots is increased. As a result, scuffing occurs on an outer circumferential surface of the punch 8 and on an inner wall surface of the hole 9 of the upper die 4 respectively.
Since the clearance A is provided, the upper die 4 and the lower die 3 are assembled in a state of involving any centering deviation within a range of the clearance A. Further, if any unbalanced load is applied in the lateral direction to the punch 8 during the forging, the pressurizing force is applied to the forging material 6 in a state in which the punch 8 is deviated in the lateral direction within the range of the clearance A.
As described above, the clearance A causes the centering deviation between the punch 8 and the cavity 7 which is formed by the upper and lower dies 4, 3. For example, when an outer cup for constructing a constant velocity universal joint is formed by forging, an inconvenience arises in that the axis of a cup of the outer cup is not coincident with the axis of a shaft thereof, i.e., the centering deviation occurs.
In the case of the cold forging die 1 described above, a clearance (not shown) is formed during the forming process at a joining surface (dividing surface) 10 (see FIG. 40) between the upper die 4 and the lower die 3. It is feared that any burr is formed by the plastically deformed forging material 6 which has entered the clearance (not shown).
An unillustrated clearance for assembling is provided at a joined section 11 based on a step section between the upper die 4 and the lower die 3 (see FIG. 39). As a result, the upper die 4 and the lower die 3 are assembled in a state in which they involve centering deviation within a range of the clearance. Therefore, there is a fear that it is possible to highly accurately maintain the coaxial degree of a product obtained by the forging, for example, the coaxial degree between the shaft and the cup of the outer cup for constructing the constant velocity universal joint.
The present applicant has already suggested a method for cooling a die which makes it possible to greatly enhance the cooling effect and improve the service life of the die (life of the die) (see Japanese Laid-Open Patent Publication No. 61-255737).
That is, as shown in FIGS. 41 and 42, the following method has been adopted. A predetermined amount of a lubricant is allowed to flow into a cavity 21 from the top. The lubricant is discharged from the cavity 21 via an outflow groove 23 formed on a knockout pin 22. After that, air is introduced into the cavity 21 via a passage 24 which communicates with the outflow groove 23. Thus, the die, which is composed of an upper die 25 and a lower die 26, is cooled. Reference numeral 27 indicates a billet after being subjected to the forging applied with a pressurizing force by the aid of a punch 28.
A mechanical press 31 as shown in FIG. 43 is generally known as a processing machine for allowing the punch to perform reciprocating motion. The mechanical press 31 is composed of a crank press having a crank mechanism, and it comprises frames 33a, 33b provided vertically on a bolster 32, a crank shaft 34 rotatably supported by the frames 33a, 33b, and a ram 36 for performing reciprocating motion in the vertical direction in accordance with the guiding action of the frames 33a, 33b by the aid of a connecting rod 35 connected to the crank shaft 34.
A material 38 is arranged on a die member 37 on the upper surface of the bolster 32. A punch 39, which is fixed to the ram 36, performs reciprocating motion in the vertical direction integrally with the ram 36 to apply a pressurizing force to the material 38. Thus, the material 38 is forged to have a predetermined shape.
In the crank press, the rotary driving force of the motor is transmitted to the crank shaft 34, and the force is converted into the reciprocating motion of the ram 36 and the punch 39 by the aid of the connecting rod 35 connected to the crank shaft 34 to generate the forming load to be applied to the material 38.
When the forging is performed by using the well known mechanical press 31 as described above, any elongation occurs, for example, in the frames 33a, 33b, the connecting rod 35, and the ram 36 for constructing the mechanical press 31. For this reason, the bottom dead center of the punch 39 which makes the vertical reciprocating motion is varied. As a result, an inconvenience arises in that any dispersion occurs in the dimension of an obtained forged product in the thickness direction.
For example, when an outer cup for constructing a constant velocity universal joint is formed by forging by using the mechanical press 31, dispersion occurs in the bottom thickness dimension of a cup of the outer cup due to the influence of the elongation.
Therefore, the conventional technique involves an inconvenience that cutting processing should be applied after the forging in order to obtain a constant bottom thickness dimension of the cup of the outer cup.
If the forming load, which is applied to the material, hugely exceeds a preset value due to the influence of the elongation (if a overloaded forming load is applied), the mechanical press 31 is stopped in a locked state, resulting in an inconvenience that it is impossible to perform continuous forming.
Further, the conventional die apparatus generally adopts a method in which a forged product is taken out by the aid of a knockout pin 43 provided movably back and forth in a hole 42 of a lower die 41. As shown in FIG. 44, a predetermined clearance C is provided between the columnar knockout pin 43 formed to have a substantially identical diameter over its outer circumferential surface and the hole 42 of the lower die 41 for inserting the knockout pin 43 thereinto.
When an outer cup for constructing a constant velocity universal joint is produced by using such a die apparatus, it is necessary to form an unillustrated centering hole at a center of one end of a shaft of the outer cup in order to apply finishing processing such as polishing. In this procedure, the centering hole is formed by mechanical processing by using a cutting tool such as a drill (including a machining center).
In view of the function of the constant velocity universal joint, the centering hole serves as a reference, for example, when grinding processing is performed for a portion for which the coaxial accuracy is required. Therefore, a high coaxial accuracy is required between the center of the centering hole and the cup and the shaft for constructing the outer cup.
However, in the case of the die apparatus concerning the conventional technique, any backlash occurs in a direction perpendicular to the axis when the knockout pin 43 makes forward and backward movement, resulting from the clearance C between the knockout pin 48 and the hole 42 of the lower die 41. Therefore, even if a projection (not shown) for forming the centering hole is provided at one end of the knockout pin 48 for constructing the die apparatus concerning the conventional technique, an inconvenience arises in that it is impossible to correctly form the centering hole at the center of one end of the shaft of the outer cup formed by the forging. As a result, the die apparatus concerning the conventional technique involves the following inconvenience. That is, the center of the centering hole formed by the projection of the knockout pin 43 is not coincident with the axis of the outer cup due the backlash as described above. Therefore, it is necessary to apply lace processing based on the reference of the centering hole to the outer circumferential surface of the outer cup formed by the forging in order to allow the center of the centering hole to coincide with the axis of the outer cup.
In this case, if the clearance C is made zero in order to avoid the backlash of the knockout pin 43, the knockout pin 43 is fastened by the hole 42, making it impossible to perform forward and backward movement. On the other hand, if the clearance C is made narrow, another inconvenience arises in that it is difficult to perform forward and backward movement of the knockout pin 48 due to the action of sliding friction with respect to the hole 42.